In circuit QED, a Rabi model can be derived by truncating the Hilbert space of an anharmonic qubit coupled to a resonator. This truncation breaks the gauge invariance present in the full Hamiltonian. Here we derive a simple criterion for an optimal gauge such that the differences between the truncated and the full Hamiltonian are minimized. We find that it is determined by the ratio of the anharmonicity of the qubit to an averaged resonator frequency. We demonstrate that the usual choices of flux and charge gauge are not necessarily the preferred options in the case of multiple resonator modes.

• Received 2 May 2019

DOI:https://doi.org/10.1103/PhysRevResearch.1.033128

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society